1. Field of the Invention
The present invention generally relates to operational amplifier current/voltage converters for instrumentation applications and, more particularly, a bias current control for solid-state amplifiers which allows the accurate measurement of very small currents.
2. Description of the Prior Art
The invention arose out of a specific application; that of a helium detector used to detect very small leaks in certain scientific equipment. The helium detector in this particular case was used to test the integrity of a linear accelerator. The interior of the accelerator was evacuated by a vacuum pump and helium was introduced to the exterior of the accelerator. A helium detector placed in the interior of the accelerator detected minute quantities, as low as 2.times.10.sup.-10 (ATM)(cc)/sec., of helium producing currents in the range of 5 to 25 femtoamperes (fA or 10.sup.-15 Ampere)
While the invention arose out of the specific application of vacuum ion gage measurement, the invention has other applications where the measurement of minute currents is required. For example, electrometer amplifiers, photodiode pre-amplifiers and mass spectrometers are other such applications that require the measurement of extremely low currents.
The measurement of current is typically performed by converting the current to a voltage which is then easily measured. One way to do this is to use an operational amplifier which is characterized by a very high output impedance. For currents in the range of 5 to 10 fA, vacuum tube amplifiers have been used because solid-state amplifiers typically cannot make accurate measurements in this range due to drift. The problem with vacuum tube amplifiers, however, is that they tend to deteriorate over time and they consume larger amounts of power.
Solid-state amplifiers have been investigated for small current measurement applications, and it has been found that the bias current can be significantly reduced by lowering the operating temperature of the amplifier. For example, as shown in the data sheets for a junction field effect transistor (JFET) input operational amplifier manufactured and sold by Analog Devices, part number AD549LH, bias current increases by a factor of 2.3 for every 10.degree. C. rise in temperature. Equation (1) predicts the value of bias current for a given temperature. EQU I.sub.B =5e.sup.-22.70(1-T/273) fA(10.sup.-15 Amperes) (1)
Equation (2) describes the change in bias current with small changes in temperature. EQU dI.sub.B =0.416e.sup.-22.70(1-T/273) fA(10.sup.-15 Amperes)(2)
Typical values of bias currents, I.sub.B, are 7.6fA at 5.degree. C., 40fA at 25.degree. C. and 319.6fA at 50.degree. C. Note that the absolute values are much reduced at lower temperature. Typical changes in bias current, dI.sub.B, per degree centigrade at various temperatures are 0.6fA/.degree. C. at 5.degree. C., 3.3fA/.degree. C. at 25.degree. C. and 26.6fA/.degree. C. at 50.degree. C.
The practical significance of this data is that if bias current is reduced by lowering the amplifier's temperature, nulling of the current becomes much easier because temperature does not have to be controlled so tightly. For example, if bias current is to be nulled to within .+-.1 fA, then temperature must be controlled as follows:
At 5.degree. C., temperature needs to be controlled to .+-.1.degree. C. for .+-.1 fA.
At 50.degree. C., temperature needs to be controlled to .+-.0.038.degree. C. for .+-.1 fA. Temperature control to .+-.1.degree. C. is not nearly so difficult as control to .+-.0.038.degree. C.
U.S. Pat No. 4,689,659 and No. 4,727,554 to Watanabe disclose temperature controllers for semiconductor devices. The approach taken by Watanabe in patent No. 4,727,554 is to mount the semiconductor device or devices on a heat sink. A Peltier cooling device is mounted to the heat sink on one side and to a heat radiating fin on the opposite side. A temperature sensor is also mounted to the heat sink. The temperature sensor generates an output current proportional to the temperature of the heat sink, and this output current is input to temperature controlling feedback circuit which controls the current to the Peltier cooling devices. In patent No. 4,689,659, Watanabe takes a similar approach except that the heat sink is replaced by a thermal insulation and the temperature sensor is mounted on the outside of the package can for the semiconductor device.
Peltier junction devices are, of course, well known in the art of cooling various electronic devices. Another example is shown in U.S. Pat. No. 4,685,081 to Richman wherein a Peltier junction device is used for thermal control of solid-state devices and, in particular, bubble memories which have a narrow temperature band of operation. In the Richman circuit, the Peltier junction is used to both heat and cool the bubble memory.
The problem of cooling the operational amplifier current to voltage converter for the helium current detector was originally addressed similarly to the temperature control used by Watanabe. The temperature control circuit controlled the temperature of the heat sink very well for this approach, measured within tolerance for controlling the amplifier's bias current. However, amplifier drift remained a problem for the very small currents which needed to be measured. This was not recognized in the prior art as represented, for example, by Watanabe, perhaps because the prior art had not attempted to measure such small currents with solid-state amplifiers. The reason for the drift was not apparent, but the fact that it persisted in spite of the cooling which was applied suggested that perhaps only vacuum tube amplifiers could, as a practical matter, be used to measure these small currents. Yet, for the particular application, a vacuum tube amplifier simply could not be used owing to the design of the apparatus.